Compact ion beam installations find broad utility in a number of manufacturing and processing applications, such as substrate cleaning by ion beam sputtering, ion-assisted deposition and etching, and various forms of surface treatment and material modification. Common constructions of ion sources for these purposes include somewhat modular ion "guns", in which a canister defines a confinement area into which an ionizable fluid such as oxygen, nitrogen or argon gas is introduced, and in which various electrodes and magnets operate to ionize and extract, and generally also accelerate, ionized particles from one end of the gun as a directed ion beam. Generally, the plasma is ionized within the chamber formed by the canister, by cathode and anode electrodes maintained at a potential difference typically of several tens of volts. A larger potential difference is usually applied to extract the ionized particles.
In one class of ion sources of this type, generally referred to as Kaufmann sources, extraction results from using two or more conductive plates having a multiplicity of small apertures in each plate, with the plates covering one wall of the plasma chamber. An inner plate serves as a conductive surface proximate to which a glowing sheet-like region of highly ionized plasma is formed. The second electrode is parallel to this apertured plate and is spaced several millimeters from it, and is maintained at a potential difference of several hundred or more volts, with apertures aligned with those of the first plate. The potential difference forms a strong electrostatic accelerating field which draws ions from the glow region within the chamber and directs them outwardly along an axis. The parallel apertured plates form a more or less homogenous electrostatic accelerating field, so that the beam extracted through each aligned pair of apertures is fairly well collimated along the projection axis.
Such ion sources are generally used within a vacuum chamber and the beam they form is directed at a workpiece or sputtering target for effecting the desired processing. The ions effectively retain their directional energy and are able to reach the target only under conditions of vacuum, e.g., 10.sup.-3 Torr or higher vacuum. In these circumstances, the volume of gas supplied to the plasma chamber to form the ions can have a substantial effect on the level of vacuum within the processing chamber. This gas is to be ionized with a high degree of efficiency, so as not to pressurize the chamber and defeat operation of the ion beam. In the foregoing Kaufmann-type constructions, the inner electrode serves, in part, to contain the low pressure plasma within the plasma chamber so that only ions of one polarity are actively drawn out by the acceleration field and leave the chamber. Other elements of these constructions, used to enhance the efficiency of ionization within the plasma chamber, include the provision of one or more magnets, and generally a cathode source for providing electrons to initiate the ionization process. Electrons emitted by the cathode follow cyclotron paths along the magnetic lines of force and are thus constrained to reside within a small area for a relatively long time. This enhances the likelihood of electrons colliding with gas molecules and forming the plasma from which ions are extracted. Once ionization is initiated, ionic conduction between the cathode and the anode can increase the number of ions available. Such current flow can also produce heat and erosion of electrodes.
Despite the added efficiencies of plasma generation achieved by physical confinement of the plasma and magnetic confinement of the electrons, the total ion current, or ion flux delivered by such a source remains limited due to the low pressure of gas necessarily employed in this setting. Furthermore, while larger currents can be achieved by scaling up multi-grid Kaufmann sources, thereby providing larger areas in which ionization occurs and a greater effective area through which ions are extracted from the plasma chamber, the nature of the processes involved, namely the provision of ionization current and the flowing of material into a plasma process, remains the subject of substantial design experimentation. Problems still arise, such as burning, unexpected creation of hot spots, erosion of components of the source, and instabilities in the beam-forming characteristics.
For processes wherein beam purity or beam collimation are not serious constraints, the design of an ion source may be relaxed somewhat, and greater absolute power densities achieved by optimizing ion-forming rather than beam forming parameters. Thus, ion treatment devices are known which produce a large flux of ionized gas, and this plasma is allowed to extend outwardly, past the front face of the plasma chamber, unconstrained by focusing or collimating grid assemblies. A suitable potential applied to the target or to a highly permeable front screen may accelerate this ion flux toward the target or workpiece. Such devices can operate with a single screen in front, preferably maintained at cathodic potential, which serves to confine electrons and yet provides a permeable opening that draws out positive ions. One such construction is shown in U.S. Pat. No. 4,710,283. In these simple constructions and in the more common Kaufmann type constructions, the front electrodes are subject to sputtering and wear. The cathodes, if separate elements, are also subject to wear. However, at least in single screen constructions, the replacement of the front electrode element is relatively straightforward, and questions of alignment and the like do not arise. The construction is thus highly suitable for bulk manufacturing processes such as etching and surface densification wherein slight sputter contamination of the beam is tolerated and beam collimation is not required.
These ion sources, less constrained by considerations of beam purity, beam collimation and beam uniformity, can produce ions at relatively high current levels. However, the presence of uncontrolled processes and hot spots remain problematic especially at high power, and there remains a need for ion source constructions which dependably generate a continuous high current plasma and emit a powerful stream of ions as a treatment beam. It is also desirable to provide a simple and rugged construction which efficiently produces ionized plasma and projects it as a beam.